A data logger is used for environmental measurement, product quality control, and the like. The data logger is measurement equipment for measuring data of a monitoring target by using a sensor or the like and saving the data. The data logger includes, for example, an RFID (Radio Frequency Identification) tag.
Examples of using a data logger include checking whether or not fresh food or frozen food is transported at a predetermined controlled temperature using a data logger which measures temperature (a temperature logger), and checking whether or not a shock is applied to a precision device during its transportation using a data logger which measures strength of shock (a shock logger).
Generally, a data logger measures data of a monitored target at as short a measurement time interval as possible in order to know a detailed state of the monitored target.
However, as the measurement time interval becomes shorter, the amount of stored data increases, thereby requiring a large-capacity storage. Hence, a logger device without a large-capacity storage, such as an RFID tag, has a problem in that its data cannot be measured at a short time interval.
Further, a data logger with a relatively large capacity also has a problem in that when stored data is transferred to a processing device such as a PC to be analyzed or viewed, the transfer time of the data is increased because the amount of the data is increased.
To solve these problems, a method of storing only abnormal data among data has been used. However, this method has a problem in that just abnormal data may be insufficient to analyze the reasons why abnormality occurs.
In patent document 1 (Japanese Patent Laid-Open No. 9-56685), an electrocardiograph which can also store normal data before and after abnormal data has been described. This electrocardiograph includes first storage means, second storage means, and control means, and operates as follows.
The first storage means stores a detected electrocardiogram waveform (corresponding to data). Then, the first storage means overwrites an electrocardiogram waveform at certain time intervals. If a detected heartbeat is different from that in normal times, the second storage means stores its electrocardiogram waveform over a certain period of time instead of the first storage means. When the second storage means finishes storing the electrocardiogram waveform, the control means transfers the electrocardiogram waveform stored in the first storage means to the second storage means as an electrocardiogram waveform before the occurrence of the abnormality,.
Therefore, the electrocardiogram waveforms before and after the occurrence of the abnormality can be stored. In addition, since the first storage means overwrites an electrocardiogram waveform at certain time intervals, the amount of stored data can be prevented from increasing.    Patent Document 1: Japanese Patent Laid-Open No. 9-56685